1. Field of the Invention
The present invention relates to a motor driver system and a motor protecting method for stably controlling a motor even when the motor is driven at a high speed or is overloaded.
2. Description of the Background Art
A motor is controlled to rotate a rotor at a desired rotation speed. That is, when the speed of rotating the rotor is observed or presumed, the motor is controlled to have the presumed speed or the measured speed to follow a reference speed. An inverter driver generates a three-phase voltage for driving the motor according to a reference voltage that varies according to the reference speed. In order to generate the reference voltage, it is common to generate a reference current.
However, the reference current or the reference voltage may be higher than a predetermined threshold value when the rotor rotates at a high speed or the motor is overloaded. In this case, it becomes difficult to control the motor.
The stability is a very important factor for a driver to drive a motor as well as performance. In general, a synchronous reluctance motor denotes a unique current angle range that generates a maximum torque, obtained based on simulation results when a motor is designed. The current angle θi denotes an angle formed between id and iq, which are values obtained by measuring current applied to the motor and converting the measured current into a synchronous coordinate (please refer to θ2 and θ2 in FIG. 4). That is, the current angle θi and the current id and iq of the motor of the synchronous coordinate have a relation described in Eq. 1.
                                          tan            ⁢                                                  ⁢                          θ              i                                =                                    i              d                                      i              q                                      ;                                  ⁢                              θ            i                    =                                    tan                              -                1                                      ⁢                                          i                d                                            i                q                                                    ;                            Eq        .                                  ⁢        1            
FIG. 1 is a block diagram illustrating a system for driving a motor according to a typical current angle control scheme.
As shown in FIG. 1, the system includes a speed controller 110 for outputting a reference current value |I*| to make a presumed speed {tilde over (ω)} of a motor to follow a reference speed ω*, a reference current generator 130 for outputting a d-axis reference current value id* and a q-axis reference current value iq* using the reference current value |I*| and a reference current angle value θ*, a reference voltage generator 140 for outputting α- and β-axis reference voltage values Vα* and Vβ* in a stationary coordinate, and an inverter 150 for generating a three-phase voltage from a direct current.
The reference current generator 130 uses a fixed reference current angle value θ*. The fixed reference current angle value is decided by obtaining a value providing the maximum efficiency at a rated speed. After the fixed reference current angle value is decided, the fixed reference current angle value is used regardless of a speed and a load.
In the typical control scheme using the fixed reference current angle value, a current angle is constant regardless of speeds and loads. Accordingly, a reference voltage or a reference current value higher than a threshold value may input when the motor is driven at a high speed or is overloaded. In this case, it is difficult to stably control the speed of the motor. In the worst case, the motor or the driver may be damaged.
FIG. 2 is a graph showing the speed-torque characteristics of a fan motor.
As shown in FIG. 2, a motor can output torque up to the maxim torque Tmax at a base speed ωb because a voltage source has enough margin. However, the maximum torque that the motor can output is gradually reduced at a speed higher than ωb. In general, a current angle is obtained from a rated torque Tnom and a rated speed ωnom, and the obtained current angle is constantly used in an entire speed domain.
Therefore, when the load is located at an overload point P where a speed is higher than the base speed ωb, it is difficult to control the motor stably by applying a reference voltage value higher than a threshold value to the motor. This is because the reference current is decided based on the current angle, and the reference voltage is decided according to the reference current.